Because of their excellent heat resistance and parting properties, organopolysiloxanes are commonly used as parting agents for copier toners. However, when dimethylpolysiloxane, a typical organopolysiloxane, is used as a toner parting agent, the methyl groups in the dimethylpolysiloxane gradually begin to be pyrolyzed in the vicinity of 150.degree. C. Pyrolysis becomes quite sudden at high temperatures over 200.degree. C., which leads to diminished parting properties and generation of gel, which causes problems with paper feed, for example.
A toner parting agent composed of a mercapto group-containing organopolysiloxane or a carboxyl group-containing organopolysiloxane has been proposed in an effort to solve these problems (Japanese Patent Publication 58-52589). A toner parting agent composed of a mixture of an amino group-containing organopolysiloxane and dimethylpolysiloxane has also been proposed (Japanese Patent Publication 59-4699). There has also been a proposal for enhancing the heat resistance of a toner parting agent by adding iron, copper, or another redox type metal or metal compound to dimethylpolysiloxane (see Japanese Laid-Open Patent Application 1-204085). These toner parting agents, however, are not entirely satisfactory, and there is a need for a parting agent for a copier toner without these problems.
One object of this invention is to provide a parting agent for a copier toner, which has no loss of parting properties or generation of gel under high-temperature conditions near 200.degree. C., and which causes no problems with paper feed or the like. These objects can be met by keeping the amount of formaldehyde generated during exposure to high temperatures within a specific range with an organopolysiloxane used as a toner parting agent.